Articles containing colorants lose their color when exposed to solar radiation for extended time periods. For example, articles placed outdoors throughout the summer often tend to display a faded version of their original color by the time autumn arrives. Although this fading occurs in both conventional and fluorescent colorants, the problem is more acute with fluorescent colorants.
The life of fluorescent colored articles is typically in the range of months when exposed to daily solar radiation, whereas the life of articles that use conventional colorants can be in the range of years. Although generally less stable, fluorescent colorants nonetheless find frequent use because of their ability to increase an article's visibility. Unlike conventional colorants, fluorescent colorants can take light that they absorb and reemit it in the visible spectrum. This innate property allows fluorescent articles to exhibit an enhanced visual contrast between the colored article and its surrounding environment.
Investigators in the retroreflective art have attempted to stabilize polymeric articles containing fluorescent colorants using various means. For example, Burns et al. in U.S. Pat. No. 5,605,761 teach the use of a hindered amine light stabilizer (HALS) to maintain the durability of articles containing fluorescent dyes in a polycarbonate polymeric matrix. The document further discloses that the fluorescent dye may be thioxanthene, perylene imide, or thioindigoid dyes, and the HALS may be compounds from the 2,2,6,6-tetraalkyl piperidine class of compounds. While these articles are extremely useful in maintaining fluorescent color stability, they are not very flexible due to the polycarbonate matrix's inherent rigidity.
Others, such as Pavelka et al. in U.S. Pat. No. 5,387,458 have attempted to maintain fluorescent colors by using an ultraviolet screening layer that screens out ultraviolet (UV) radiation in the range of 340 to 400 nanometers. The document also discloses that the fluorescent color resides in a separate layer rather than in the screening layer. Although these articles are highly beneficial because of their stable fluorescent colors, they do present the need of having two separate layers that can add cost to the construction. Furthermore, the screening layer may not be effective in reducing the degradation of the fluorescent dye caused by dye absorbtion of visible radiation.
Polyvinyl chloride (PVC) films are useful in many applications because of their flexibility and commercial availability. UV absorbing stabilizers have been commonly used in polyvinyl chloride articles to light stabilize the polymer matrix. See, e.g., Marice McMurrer, Update: UV Stabilizers, PLASTICS COMPOUNDING 40 (January/February 1985). UV stabilizers, however, are not effective in stabilizing fluorescent dyes in the matrix.
Although PVC films containing fluorescent dyes are widely available today, they tend to have very poor color retention. Factors contributing to the color fading include lack of dye solubility in the PVC host matrix, dye migration, and minimal protection offered by the resin against photodegradation.
Technical publications have suggested that HALS, with its amine group in the molecular structure, may not be compatible with PVC. For example, T. Hjertberg and E. M. Sörvik stated in Thermal Degradation of PVC, in DEGRADATION AND STABILISATION OF PVC, E. D. Owen (editor) 21, 69 (1984) that amines “induce dehydrochlorination of PVC at high temperatures” leading to degradation of the PVC matrix. In addition, HALS based on secondary or tertiary piperidinyl amines are very basic compounds. For example, 2,2,6,6-tertamethyl piperidine has a pkb of 2.9 as compared to 4,7 for ammonia when measured in water. See Can Zhang et al., Hindered Amine Light Stabilizers: Effects of Acid Exposure, Volume 24 of JOURNAL OF POLYMER SCIENCE: PART C: POLYMER LETTERS 453, 453 (1986). Because of its alkalinity, HALS in the presence of a volatile acid, such as hydrochloric acid (HCl), forms a salt. Hydrochloric acid is produced by degradation and oxidation reactions resulting from “light induced aging of PVC films.” See Martinez et al., Prediction of Photoageing Stability of Plasticized PVC Films Containing UV-Stabilisers, Volume 54 of POLYMER DEGRADATION AND STABILITY 49, 49 (1996). The presence of a basic HALS in combination with a readily available source of HCl gives rise to acid-base reactions that can degrade the PVC matrix.
Because of the flexible nature of PVC films and the desirability using of fluorescent colorants in many articles, there is a need for a durable colored article having these combinations.